Projector

ABSTRACT

A projector includes a light source unit that supplies light, and a spatial light modulator that modulates light supplied from the light source unit according to an image signal. The spatial light modulator is driven by an applied voltage whose polarity is reversed according to a polarity-reversing frequency which is specific to the spatial light modulator. The light source unit supplies light which is modulated according to pulse width modulation for which fundamental frequency is set based on the polarity-reversing frequency.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Continuation of U.S. patent application Ser. No. 11/812,054filed Jun. 14, 2007, which claims the benefit of Japanese PatentApplication No. 2006-187526 filed on Jul. 7, 2006. The disclosures ofthe prior applications are hereby incorporated by reference herein intheir entireties.

BACKGROUND

1. Technical Field

The present invention relates to a projector, and more particularly to atechnique concerning a projector which includes a liquid crystal displayas a spatial light modulator.

2. Related Art

Projectors display images by projecting light modulated according toimage signals. Conventionally, techniques have been proposed formodulating the light before the light comes into a spatial lightmodulator of the projector, for example, in addition to modulating thelight in the spatial light modulator (see JP-A-2004-354717, forexample). When the incoming light to the spatial light modulator ismodulated, the projector can display the image in a wider dynamic rangethan a dynamic range corresponding to the control of the spatial lightmodulator.

As a spatial light modulator, a liquid crystal display can be employed,for example. To prevent degradation of liquid crystal material called“image persistence”, the polarity of an applied voltage to the liquidcrystal display is reversed and the resulting alternate-current voltagewith the reversed polarity is applied to drive the liquid crystaldisplay every predetermined time period. When the polarity of theapplied voltage is reversed, in other words, when the applied voltagechanges its polarity from the positive to the negative, a displayedimage sometimes changes its brightness slightly. Even if there is aslight change in the brightness, a viewer recognizes the image in anaverage brightness as far as a light source unit is constantly on,because in this case the image corresponding to the positive polarityand the image corresponding to the negative polarity are integratedwithout bias. On the other hand, when the ON-OFF switching of the lightsource unit is controlled based on Pulse Width Modulation (PWM), forexample, the image may be displayed for different time periodscorresponding to the positive polarity and to the negative polarity. Inthis case, since the images of the negative polarity and the positivepolarity are not equally integrated, the change in brightness becomesrecognizable, causing problems such as gradation shift andnon-uniformity of display. Then, high-quality image display is difficultto achieve. As can be seen, techniques as described above havedifficulties in displaying high-quality images in a wide dynamic range.

SUMMARY

An advantage of some aspects of the invention is that a projector candisplay high-quality images in a wide dynamic range.

A projector according to an aspect of the invention includes a lightsource unit that supplies light, and a spatial light modulator thatmodulates light supplied from the light source unit according to animage signal. The spatial light modulator is driven by an appliedvoltage whose polarity is reversed according to a polarity-reversingfrequency which is specific to the spatial light modulator. The lightsource unit supplies light which is modulated according to pulse widthmodulation for which fundamental frequency is set based on thepolarity-reversing frequency.

It is preferable that at least one of (A) the fundamental frequency isan even multiple of the polarity-reversing frequency, and (B) a phase ofthe pulse width modulation is reversed every polarity-reversing periodaccording to the polarity-reversing frequency, be satisfied.

It is preferable that the phase of the pulse width modulation bereversed every time one writing of the image signal is performed for anentire screen.

It is preferable that the light source unit supply light according to asmoothed signal which is obtained by smoothing a pulse width modulation(PWM) signal.

It is preferable that there be plural light source units, and that thelight source units be controlled according to difference in outputsthereof.

It is preferable that the light source unit include a solid-state lightsource.

It is preferable that the spatial light modulator include a liquidcrystal display.

The above and other objects, features, advantages and technical andindustrial significance of this invention will be better understood byreading the following detailed description of presently preferredembodiments of the invention, when considered in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1 is a schematic diagram of a structure of a projector according toone embodiment of the invention.

FIG. 2 is a schematic diagram illustrating a manner of driving a spatiallight modulator.

FIG. 3 is a schematic diagram illustrating an ON/OFF switching of anLED.

FIG. 4 is a schematic diagram illustrating non-uniformity of display.

FIG. 5 is a schematic diagram of a case where one-image writing periodis ½ of a polarity-reversing period.

FIG. 6 is a schematic diagram of a relation between polarity reversaland driving of LED in the embodiment of the invention.

FIG. 7 is a schematic diagram of ON-time corresponding to positivepolarity and ON-time corresponding to negative polarity.

FIG. 8 is a schematic diagram of another example of fundamentalfrequency setting.

FIG. 9 is a schematic diagram of still another example of fundamentalfrequency setting.

FIG. 10 is a schematic diagram of a comparative example of theembodiment.

FIG. 11 is a block diagram of a structure of elements for driving theprojector.

FIG. 12 is a schematic diagram illustrating a response characteristic ofa liquid crystal.

FIG. 13 is a schematic diagram illustrating non-uniformity of brightnessattributable to the response characteristic of a liquid crystal.

FIG. 14 is a schematic diagram illustrating non-uniformity of brightnessin a moving picture.

FIG. 15 is a schematic diagram of a control operation for reducing thenon-uniformity of brightness.

FIG. 16 is a block diagram of a modification of the structure ofelements for driving the projector.

FIG. 17 is a schematic diagram of a smoothed signal supplied from anLPF.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Exemplary embodiments of the invention will be described in detail belowwith reference to the accompanying drawings.

FIG. 1 is a schematic diagram of a projector 10 according to oneembodiment of the invention. The projector 10 is a front projector whichprojects light on a screen 18 so that the viewer can observe lightreflected from the screen 18 as images. A red-light (R-light) lightemitting diode (LED) 11R is a solid-state light source that serves as alight source unit which supplies R-light. The R-light emitted from theR-light LED 11R is turned into parallel light in a collimator lens 12before entering into an R-light spatial light modulator 13R. The R-lightspatial light modulator 13R is a transmissive liquid crystal displaywhich modulates the R-light according to image signals. The R-lightspatial light modulator 13R emits modulated R-light which enters a crossdichroic prism 14 which serves as a color composite optical system.

A green-light (G-light) LED 11G is a solid-state light source thatserves as a light source unit which supplies G-light. The G-lightemitted from the G-light LED 11G is turned into parallel light in thecollimator lens 12 before entering into a G-light spatial lightmodulator 13G. The G-light spatial light modulator 13G is a transmissiveliquid crystal display which modulates the G-light according to imagesignals. The G-light spatial light modulator 13G emits modulated G-lightwhich enters the cross dichroic prism 14 from a different side from aside through which the R-light enters.

A blue-light (B-light) LED 11B is a solid-state light source that servesas a light source unit which supplies B-light. The B-light emitted fromthe B-light LED 11B is turned into parallel light in the collimator lens12 before entering into a B-light spatial light modulator 13B. TheB-light spatial light modulator 13B is a transmissive liquid crystaldisplay which modulates the B-light according to image signals. TheB-light spatial light modulator 13B emits modulated B-light which entersthe cross dichroic prism 14 from a different side from the side throughwhich the R-light or the G-light enters. The projector 10 may include anequalizing optical system which equalizes intensity distribution of alight flux. For example, the projector 10 may include a rod integratoror a fly's eye lens.

The cross dichroic prism 14 includes two dichroic films 15 and 16 whichare arranged so as to be substantially perpendicular to each other. Thefirst dichroic film 15 reflects the R-light and transmits the G-lightand the B-light. The second dichroic film 16 reflects the B-light andtransmits the R-light and the G-light. The cross dichroic prism 14combines the R-light, G-light, and B-light coming in from differentsides and emits the resulting composite light in a direction of aprojection lens 17. The projection lens 17 receives and projects thelight combined by the cross dichroic prism 14 onto the screen 18.

FIG. 2 is a schematic diagram illustrating how the spatial lightmodulators 13R, 13G, and 13B, which are liquid crystal displays, aredriven. When the liquid crystals are driven, if a direct-current voltageof the same polarity is applied for an extended period of time, imagepersistence, which is degradation of liquid crystals, occurs. To preventthe image persistence, the spatial light modulators 13R, 13G, and 13Bare driven by alternate-current voltage. For example, assume that theliquid crystal display has a specific frequency of 60 Hz for reversingthe polarity (hereinafter referred to as “polarity-reversingfrequency”). In this case, the spatial light modulators 13R, 13G, and13B are driven by an alternate-current voltage with a polarity-reversingperiod T of 1/60 second. Here, the polarity-reversing period T is aperiod according to which the polarity of an applied voltage isreversed. If a time period set for one-image writing is 1/60 second,i.e., the same as the polarity-reversing period T, the polarity isreversed during the one-image writing period. After a writing positionL1 of positive polarity passes a pixel, the positive potential isretained in the pixel until the next writing position L2 of negativepolarity comes to the pixel. After the writing position L2 of negativepolarity passes the pixel, the negative potential is retained in thepixel until the next writing position L1 of positive polarity reachesthe pixel.

The brightness of the image may change slightly according to thepolarity of the applied voltage. However, when the LEDs 11R, 11G, and11B are constantly on, the image of the positive polarity and the imageof the negative polarity are integrated within one-image writing periodwithout bias. Therefore, even if there is a slight difference in thebrightness according to the polarity of the applied voltage, the viewerrecognizes an image in the average brightness.

FIG. 3 is a schematic diagram illustrating ON/OFF switching of the LEDs11R, 11G, and 11B based on PWM. The LEDs 11R, 11G, and 11B arecontrolled based on PWM. The PWM control allows for stable and easycontrol using a digital circuit. The viewer recognizes an integration ofimages displayed during the ON-time of the LEDs 11R, 11G, and 11B as animage. Therefore, if the brightness of the images are adjusted based onthe ON-time of the LEDs 11R, 11G, and 11B, the images can be displayedin a wider dynamic range than the dynamic range corresponding to thecontrol of the spatial light modulators 13R, 13G, and 13B.

When the ON/OFF switching of the LEDs 11R, 11G, and 11B are performedbased on PWM, a time the image is displayed corresponding to thepositive polarity sometimes differs from a time the image is displayedcorresponding to the negative polarity. For example, assume that theLEDs 11R, 11G, and 11B are turned ON and OFF three times during thepolarity-reversing period T, as shown in FIG. 3. In this case, if theimages displayed during ON-time of the LEDs 11R, 11G, and 11B areintegrated, brightness corresponding to the positive polarity isdominant in some portions of the resulting image, while brightnesscorresponding to the negative polarity is dominant in other portions, asshown in FIG. 4. As a result, three sets of a positive-polarity-dominantportion and a negative-polarity-dominant portion are displayed, and theviewer recognizes non-uniformity of display as three belt-like portions.

FIG. 5 is a schematic diagram of a case where the one-image writingperiod is set to 1/120 second which corresponds to ½ of thepolarity-reversing period T. In this case, the polarity is reversed notduring the one-image writing period. Instead, the polarity is reversedevery time the one-image writing is finished. Similarly to the caseshown in FIG. 3, the LEDs 11R, 11G, and 11B are turned ON and OFF threetimes during the polarity-reversing period T. When the images displayedin the ON-time of the LEDs 11R, 11G, and 11B are integrated, thebrightness of the entire resulting image is biased to the brightnesscorresponding to the positive polarity. In this case, dissimilar to thecase shown in FIG. 4, there is no non-uniformity in the displayed image.However, the viewer can recognize the gradation shift. When thedisplayed images has non-uniformity or gradation shift due to the mannerof control of the LEDs 11R, 11G, and 11B, high-quality image displaycannot be achieved even if the image can be displayed in a wide dynamicrange.

The projector 10 according to the embodiment of the invention sets afundamental frequency of PWM based on the polarity-reversing frequencyso that a time period the LEDs 11R, 11G, and 11B are turned ON by thepositive polarity is equal to a time period the LEDs 11R, 11G, and 11Bare turned ON by the negative polarity. When the fundamental frequencyof PWM for the LEDs 11R, 11G, and 11B is represented as fp, thepolarity-reversing frequency for the spatial light modulators 13R, 13G,and 13B is represented as fr, and an arbitrary positive integer isrepresented as n, at least one of following expressions (1) and (2) issatisfied:

Error! Objects cannot be created from editing field codes.  (1),

fp=(n+½)×fr  (2).

The expression (1) indicates that the fundamental frequency fp of PWM isan even multiple of the polarity-reversing frequency fr for the spatiallight modulators 13R, 13G, and 13B. The expression (2) indicates thatthe phase of PWM is reversed every polarity-reversing period.

FIG. 6 is a schematic diagram illustrating a relation between thereversing of the polarity and the driving of the LEDs 11R, 11G, and 11Baccording to the embodiment of the invention. Here, thepolarity-reversing frequency fr is set to 60 Hz. In the case of FIG. 6,the fundamental frequency fp of PWM whose period consists of oneON-period and one OFF-period is set to 120 Hz. Therefore, thefundamental frequency fp is twice the polarity-reversing frequency fr,whereby the expression (1) is satisfied (n=1). In this case, the ON-timecorresponding to the positive polarity and the ON-time corresponding tothe negative polarity are equal within the polarity-reversing period T,as shown in FIG. 7. Since the persistent images of both the imagecorresponding to the positive polarity and the image corresponding tothe negative polarity are integrated without bias, the difference inbrightness of the images corresponding to the positive polarity and thenegative polarity can be offset with each other. The same effect asobtained when n=1 can be obtained when n=2 in the above expression (1).In FIG. 6, the ON-time and the OFF-time each occupy a half (50%) of afundamental period, i.e., 1/fp second, of the PWM. However, the sameeffect as mentioned above, i.e., offsetting of the brightnessdifference, can be obtained when the ON-time and the OFF-time aredetermined as appropriate so as to occupy other fractions of the period.

FIGS. 8 and 9 are schematic diagrams illustrating cases where thefundamental frequency fp is set to a different value. In the case shownin FIG. 8, the fundamental frequency fp is 90 Hz, whereby the expression(2) is satisfied (n=1). When the phase of PWM changes from ON, OFF, toON during one polarity-reversing period T, in the next period T, thephase changes from OFF, ON, to OFF. Further, in the subsequent period T,the phase changes from ON, OFF, to ON, thereby returning to the originalphase. In this case, the length of the ON-time corresponding to thepositive polarity becomes equal to the length of the ON-timecorresponding to the negative polarity within two polarity-reversingperiods (T×2). Thus, since the images corresponding to the positivepolarity and the negative polarity are integrated as persistent imageswithout bias, the brightness difference between the images correspondingto the positive polarity and the negative polarity can be offset.

In the case of FIG. 9, the fundamental frequency fp is 150 Hz, wherebythe expression (2) is satisfied (n=2). When the phase of PWM changesfrom ON, OFF, ON, OFF, to ON in one polarity-reversing period T, in thenext period T, the phase changes from OFF, ON, OFF, ON, to OFF. Furtherin the next period T, the phase of PWM returns to the original phases,i.e., ON, OFF, ON, OFF, to ON. In this case, the length of the ON-timecorresponding to the positive polarity becomes equal to the length ofthe ON-time corresponding to the negative polarity within twopolarity-reversing periods (T×2). Therefore, the brightness differencein images corresponding to the positive polarity and the negativepolarity can be offset with each other. Thus, the same effect asobtained when n=1 or 2 can be obtained when n is equal to or larger than3 in the expression (2).

FIG. 10 is a schematic diagram of a comparative example for theembodiment where neither the expression (1) nor the expression (2) issatisfied. In the example shown in FIG. 10, the fundamental frequency fpis set to 180 Hz. When fp=180 Hz, the expressions (1) and (2) are notsatisfied. In order to make the viewer recognize the image in theaverage brightness obtained by integration of the image corresponding tothe positive polarity and the image corresponding to the negativepolarity, it is desirable that the difference in brightness be offsetwithin approximately two polarity-reversing periods. In the case shownin FIG. 10, the time the image is displayed in the positive polarityexceeds the time the image is displayed in the negative polarity in twopolarity-reversing periods (T×2). Therefore, the viewer ends uprecognizing the image in brightness biased to the brightnesscorresponding to the positive polarity.

As can be seen from the foregoing, the satisfaction of at least one ofthe expressions (1) and (2) means offsetting of the brightnessdifference of the images corresponding to the positive polarity and thenegative polarity within at least two polarity-reversing periods.Offsetting of the difference in brightness of the images allows fordisplay of high-quality images where gradation shift and non-uniformityare reduced. Thus, the high-quality images can be displayed with a widedynamic range. In consideration of the influence of noises to thecontrol signals, the integer n in the expressions (1) and (2) isdesirably a number equal to or larger than five. For example, n can beapproximately 100. Though FIGS. 6 to 9 illustrate the examples where thepolarity is reversed within the one-image writing period, the polaritymay be reversed every time the one-image writing is finished, ratherthan within the one-image writing period.

FIG. 11 is a block diagram of elements for driving the projector 10. AnAD converter 20 converts an image signal, which is supplied fromexternal equipment and the like in an analog form, into a digitalsignal. A DSP(1) 21 which is a digital signal processing circuitextracts a brightness parameter from the image signal converted into adigital form for controlling the brightness of the image. A DSP(2) 22which is a digital signal processing circuit converts the brightnessparameter extracted by the DSP(1) 21 into a duty cycle of PWM. A Look UpTable (LUT) can be employed for the conversion of the brightnessparameter into the duty cycle.

The DSP(2) 22 also performs conversion of the duty cycle according to alight-source control signal directly supplied to the DSP(2) 22 fromoutside. The light-source control signal directly supplied to the DSP(2)22 is previously set according to output difference of respective LEDs11R, 11G, and 11B. For example, if the ON-time of the R-light LED 11R isto be adjusted to be 80% in maximum, the DSP(2) 22 performs an operationto multiply the output of the LUT by 80%. When the intensity of lightsupplied from each of the LEDs 11R, 11G, and 11B is adjusted, apreferable white balance can be achieved. When plural LEDs are employedfor each color of light, the light-source control signal can be employedto control the LEDs for the same color according to their difference inoutput.

A PWM signal generating unit 26 generates a PWM signal by modulating apulse width based on the output from the DSP(2) 22. Fundamentalfrequency of the PWM signal generated by the PWM signal generating unit26 is determined according to the polarity-reversing frequency. An LEDdriving unit 27 drives the LEDs 11R, 11G, and 11B according to the PWMsignal supplied from the PWM signal generating unit 26. The LEDs 11R,11G, and 11B supply the light modulated according to the PWM signal.Thus, the LEDs 11R, 11G, and 11B are controlled based on the imagesignal and the light-source control signal.

A DSP(3) 23 which is a digital signal processing circuit expands thegradation range of image signals based on the brightness parameterextracted by the DSP(1) 21. The expansion of the gradation range allowsfor display of high-contrast images which makes the best of the dynamicrange of the spatial light modulators 13R, 13G, and 13B. After the imagesignal is subjected to the expansion processing in the DSP(3) 23, an DAconverter 24 converts the expanded image signal into an analog form. Aspatial-light-modulation driving unit 25 drives the spatial lightmodulators 13R, 13G, and 13B according to the image signal convertedinto an analog form. The spatial light modulators 13R, 13G, and 13Bmodulate the light emitted from the LEDs 11R, 11G, and 11B,respectively, according to the image signal. The structure of theprojector 10 is not limited to a structure which adjusts the lightintensity based on the brightness parameter of the image signal and thelight-source control signal. The projector 10 may be configured so as toadjust the light intensity based on one of the brightness parameter andthe light-source control signal.

FIG. 12 is a graph of a response characteristic of liquid crystals inthe liquid crystal display. Specifically, FIG. 12 shows a relationbetween time t elapsed since the start of voltage application andintensity I of light transmitted through the liquid crystal layer. Theliquid crystal completes its orientational change in response to theapplied voltage when a predetermined time t1 elapses since the start ofvoltage application, and transmits the light of intensity I1corresponding to the applied voltage. Before the predetermined time t1elapses from the start of voltage application, the intensity I oftransmitted light is different from the intensity I1 corresponding tothe applied voltage, since the liquid crystal has not completed itschange into an oriented state corresponding to the applied voltage.Thus, the liquid crystal display needs a response time after receivingthe signal until giving a response.

FIG. 13 is a schematic diagram illustrating non-uniformity of brightnessattributable to the response characteristic of the liquid crystal. Forexample, assume that the entire screen is switched from black to whiteduring one-image writing period. The one-image writing period is ascanning period during which one writing is performed on all scanningelectrodes. Writing position L3 for displaying white scans all scanningelectrodes sequentially during a scanning period S. Since the sequentialscanning of the writing position L3 is performed on the liquid crystaldisplay all of whose pixels display black, an area AR0 where the writingposition L3 has not passed remains in black. On the other hand, an areaAR2 where the response time of the liquid crystal has passed since thepassage of the writing position L3 is completely in white. In an areaAR1 where the response time of the liquid crystal has not passed sincethe passage of the writing position L3, the switching from black towhite is not complete. Therefore, in the area AR1, gradation appears insuch a manner that the black color gradually changes to the white coloraccording to the distance from the writing position L3.

When the entire screen is left in white after the switching from blackto white, the screen comes to a stable state displaying white, wherebyinstantaneous cancellation of the gradation is possible. Therefore, inthe static image display, the viewer hardly recognizes thenon-uniformity of brightness attributable to the response characteristicof the liquid crystal. On the contrary, in moving picture display, wherean image of a different brightness from the brightness of backgroundmoves in the screen, for example, the viewer sometimes notices thenon-uniformity of brightness caused by the response characteristic ofthe liquid crystal. For example, when a white image moves in a directionof arrow against the black background as shown in FIG. 14,non-uniformity of brightness sometimes becomes visible in an area arounda boundary M1 where black is being switched to white and an area arounda boundary M2 where white is being switched to black.

When the LEDs 11R, 11G, and 11B are left ON constantly, the displaycomes to a stable state displaying white or black immediately after thepassage of the boundaries M1 or M2. Therefore, it is possible to makegradation difficult to recognize. However, when the ON/OFF switching ofthe LEDs 11R, 11G, and 11B is performed by PWM, an area where theresponse of the liquid crystals is not completed sometimes appears as ifhighlighted, so as to make gradation easily noticed. For example, whenthe boundary M1 passes a pixel, the color is changed from black to whitesimilarly to the case shown in FIG. 13. If the PWM continues so that thephase is OFF in the first half of the one-image writing period and ON inthe latter half, a similar gradation as shown in FIG. 13 comes to behighlighted.

Further, at a pixel where the boundary M2 passes, the color is changedfrom white to black, contrarily to the case of FIG. 13. In this case,though the area where the response time of liquid crystal has passedafter the writing position passes exhibits black, an area close to thewriting position remains in white. Therefore, if PWM continues so thatthe former half is OFF and the latter half is ON in the one-imagewriting period, gradation which is vertically reversed from thegradation shown in FIG. 13 comes to be highlighted. The non-uniformityof brightness appearing as gradation can disturb image viewing.

FIG. 15 is a timing chart of a control operation for alleviating thenon-uniformity of brightness attributable to the response characteristicof the liquid crystal. In the case shown in FIG. 15, the fundamentalfrequency fp of PWM is 330 Hz, which satisfies the expression (2) (n=5).The pulse width of the PWM is determined based on a fundamental period P(= 1/330 second) as a maximum pulse width. The polarity-reversing periodT is 1/60 second. When a scanning frequency for writing of all scanningelectrodes is represented as fs, a following expression (3) issatisfied:

fp=(n+½)×fs  (3).

The expression (3) indicates that the phase of the PWM is reversedwhenever the image signal writing corresponding to one image isperformed on the entire screen. In the example shown in FIG. 15, thescanning period S for performing the writing once for all the scanningelectrodes is set to 1/60 second, which is the same as thepolarity-reversing period T. The scanning frequency fs is 60 Hz, whichis the same as the polarity-reversing frequency fr, whereby theexpression (3) is satisfied (n=5).

When the phase of the PWM is reversed every scanning period S, the imagewith the gradation and the image in white are integrated in an area nearthe boundary M1 shown in FIG. 14, for example. Though some blurring mayoccur due to integration of the image with gradation and the image inwhite, unnatural gradation obstructing a comfortable image viewing canbe reduced. Similarly, in an area around the boundary M2, the image withgradation and the image in black may be integrated so that the gradationbecomes less noticeable. Thus, the non-uniformity in brightnessparticularly in the moving picture can be reduced.

When both the polarity-reversing frequency fr and the scanning frequencyfs are 60 Hz, the fundamental frequency fp which satisfies theexpressions (2) and (3) are: 90 Hz, 150 Hz, . . . , and [(n+½)×60]Hz.When the polarity-reversing frequency fr and the scanning frequency fsare set to the same value, it is possible to determine the fundamentalfrequency fp in such a manner that the non-uniformity of display causedby reversing the polarity and non-uniformity of brightness attributableto the response characteristic of liquid crystals can be reducedsimultaneously.

FIG. 16 is a block diagram of a modification of the structure fordriving the projector 10. The modification is characterized in that itincludes a low-pass filter (LPF) 30. The LPF 30 generates a smoothedsignal by smoothing a PWM signal supplied from the PWM signal generatingunit 26, and supplies the smoothed signal to the LED driving unit 27.The LEDs 11R, 11G, and 11B supply light corresponding to the smoothedsignal supplied from the LPF 30. The LPF 30 can be implemented as an RCcircuit or the like. The LPF 30 may be embedded into the PWM signalgenerating unit 26 or the LED driving unit 27.

FIG. 17 is a graph of a smoothed signal supplied from the LPF 30. TheLPF 30 smoothes the signal by cutting peak portions, i.e., portions overa reference value, from the PWM signal indicated by a dotted line andsupplementing bottom portions. If phase shift occurs in the PWM signalrelative to the clock, an influence of the phase shift becomesnoticeable as the difference in brightness between the image of positivepolarity and the image of negative polarity increases. The use of theLPF 30 in the smoothing of the PWM signal realizes precisecorrespondence between the driving of the spatial light modulators 13R,13G, and 13B, and the driving of the LEDs 11R, 11G, and 11B, therebyenhancing the robustness against the phase shift of the PWM signals.

The projector 10 is not limited to those employing the transmissiveliquid crystal display as the spatial light modulator. The projector 10may employ a reflective liquid crystal display. The projector 10 is notlimited to a front projector. The projector 10 may be a rear projectorwhich supplies light to one surface of a screen so that the viewer viewslight passing through the screen and emitted from the other surface ofthe screen as an image. Further, the invention is applicable to adirect-vision display using a liquid crystal display.

According to the embodiments, the fundamental frequency of PWM can beset based on the polarity-reversing frequency in such a manner that atime the light source unit is turned on by the positive polarity isequal to a time the light source unit is turned on by the negativepolarity. Since the image corresponding to the positive polarity and theimage corresponding to the negative polarity are integrated aspersistent images without bias, the difference in brightness between theimages corresponding to the positive polarity and the negative polaritycan be offset. Offsetting of the difference in brightness of the imagesallows for a display of high-quality images in which the gradation shiftand the non-uniformity of display are reduced. Thus, a projector candisplay high-quality images in a wide dynamic range.

In the embodiments, the fundamental frequency is an even multiple of thepolarity-reversing frequency, or a phase of the pulse width modulationis reversed every polarity-reversing period according to thepolarity-reversing frequency. Further, the difference in brightness isoffset within approximately two polarity-reversing periods in order tomake the viewer recognize the image in average brightness obtainedthrough integration of the image corresponding to the positive polarityand the image corresponding to the negative polarity. When thefundamental frequency of PWM is an even multiple of thepolarity-reversing frequency, the difference in brightness can be offsetwithin one polarity-reversing period. When the phase of PWM is reversedevery polarity-reversing period, the difference in brightness can beoffset within two polarity-reversing periods. Thus, the difference inbrightness between the images corresponding to the negative polarity andthe positive polarity can be offset.

Further, in the embodiments, the phase of the pulse width modulation maybe reversed every time the writing of the image signal is performed oncefor the entire screen. The liquid crystal display requires a certainresponse time after the signal writing until the liquid crystalmolecules are turned into a state corresponding to the applied voltage.When the light source unit is turned on or turned off during one-imagewriting, the time elapsed since the signal writing may be different foreach liquid crystal molecule depending on the time the light source unitis turned on and off, sometimes resulting in non-uniformity ofbrightness in the displayed image. When the phase of PWM is reversedevery time the one-image writing is finished, a portion where theresponse of liquid crystals is not complete can be made less noticeable.Thus, non-uniformity in brightness particularly in a moving picture canbe reduced.

Further, in the embodiments, the light source unit supplies the lightaccording to the smoothed signal which is obtained by smoothing thepulse width modulation (PWM) signal. Then, even if the phase shiftoccurs in the PWM signal, influence thereof can be reduced and a precisedisplay can be achieved.

In the embodiments, there are plural light source units and the lightsource units are controlled according to difference in outputs thereof.Therefore, the control can be achieved corresponding to the differencein outputs of the light source units. Further, when the adjustment isperformed based on the difference in outputs of the light source unitscorresponding to each color, a preferable white balance can be achieved.

In the embodiments, the solid-state light source which can be turned onand off at high speed is employed. Therefore, it is possible to supplythe light modulated by PWM correctly.

In the embodiments, the liquid crystal display is employed as thespatial light modulator. When the spatial light modulator is a liquidcrystal display, the light modulation can be performed in accordancewith the image signals. The liquid crystal display is driven by anapplied voltage whose polarity is reversed according to a predeterminedpolarity-reversing frequency. Thus, in a structure including the liquidcrystal display, the gradation shift and the non-uniformity of displayattributable to the polarity-reversed driving can be reduced, whereby ahigh-quality image can be displayed in a wide dynamic range.

As can be seen from the foregoing, the projector according to theinvention is suitable for the use with a solid-state light source and aliquid crystal display.

Although the invention has been described with respect to a specificembodiment for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art that fairly fall within the basic teaching herein setforth.

What is claimed is:
 1. A projector comprising: a light source unit thatsupplies light; and a spatial light modulator that modulates lightsupplied from the light source unit according to an image signal,wherein the spatial light modulator is driven by an applied voltagewhose polarity is reversed according to a polarity-reversing frequencywhich is specific to the spatial light modulator, and the light sourceunit supplies light which is modulated according to pulse widthmodulation for which fundamental frequency is set based on thepolarity-reversing frequency.
 2. The projector according to claim 1,wherein at least one of (A) the fundamental frequency is an evenmultiple of the polarity-reversing frequency, and (B) a phase of thepulse width modulation is reversed every polarity-reversing periodaccording to the polarity-reversing frequency, is satisfied.
 3. Theprojector according to claim 2, wherein the phase of the pulse widthmodulation is reversed every time one writing of the image signal isperformed for an entire screen.
 4. The projector according to claim 1,wherein the light source unit supplies light according to a smoothedsignal which is obtained by smoothing a pulse width modulation signal.5. The projector according to claim 1, wherein there are plural lightsource units, and the light source units are controlled according todifference in outputs of the light source units.
 6. The projectoraccording to claim 1, wherein the light source unit includes asolid-state light source.
 7. The projector according to claim 1, whereinthe spatial light modulator includes a liquid crystal display.